NEW YORK (Reuters) — Older women who drink two or more cups of coffee a day, or four cans of caffeinated cola daily may be lowering their risk for breast cancer, while at the same time upping their chances of developing osteoporosis, a new study on caffeine and sex hormones suggests.

Reporting in the October issue of the American Journal of Epidemiology, researchers say these changes in disease risk may be associated with decreased levels in some types of estrogen, and a decrease in the male sex hormone testosterone in postmenopausal women who consume a lot of caffeine.

According to Drs. Rebecca Ferrini and Elizabeth Barrett-Conner of the University of California-San Diego, in La Jolla, their three-year study of 728 Caucasian women, aged 42 to 90 years, showed that the blood of high-caffeine consumers had increased amounts of a substance — sex hormone-binding globulin — produced by the liver that sweeps up and metabolizes steroid hormones, such as testosterone and estradiol, a form of estrogen.

However, the researchers did not find a similar drop in estradiol, which they say is probably because levels of this estrogen hormone are already very low in women who have gone through menopause.

Estradiol is the most important of the female sex hormones, and is essential for breast development and the health of the reproductive system.

“These intriguing caffeine-hormone associations may explain some of the previously reported associations between hormone-related conditions and the intake of caffeinated beverages,” they write.

“If caffeine increases sex hormone-binding globulin levels, this may result in decreased levels of bioavailable estradiol and testosterone, providing one possible mechanism for both a diminished breast cancer risk, and increased risk of osteoporosis among caffeine users,” the authors conclude.

But, they also note that their finding of high levels of estrone — another estrogen hormone — among high-caffeine users “may partially account for the association between caffeine consumption and endometriosis” (inflammation of the uterus lining) found in studies of younger, premenopausal women.